1. Field of the Invention
The present invention relates to a light valve apparatus, and a projection display system and a view-finder system employing said light valve apparatus.
Conventionally, there has been known a system in which optical images corresponding to video signals are formed on a light valve, and light is irradiated onto the optical images for projection thereof onto a large screen through magnification by a projection lens. Recently, there has been disclosed a projection display unit which employs a liquid crystal panel as a light valve, for example, in U.S. Pat. No. 5,042,929 to Tanaka et al., to which attention has been directed from the viewpoint that the projection unit may be made compact as a whole.
In order to obtain a projection image at high image quality, it has been a main tendency that the liquid crystal panel employs twist nematic (TN) liquid crystal as the material, and TFT (thin film transistors) are used for respective pixels (i.e., picture elements) as switching elements so as to adopt an active matrix type, with three sheets of liquid crystal panels being used for red, green and blue respectively.
FIG. 23 shows one example of conventional constructions of an optical system for a projection display system employing the liquid crystal panel.
In the known arrangement of FIG. 23, light emitted from a light source 11 is incident upon a color separating optical system constituted by dichroic mirrors 12 and 13, and a flat mirror 14 so as to be separated into light rays in three primary colors of red, green and blue. Each primary light color passes through incident side polarizing plates 18, 19 and 20 after having been transmitted through field lenses 15, 16 and 17 and is incident upon the liquid crystal panels 21, 22 and 23. The optical images formed as variations of rotatory polarization in the liquid crystal panels 21, 22 and 23 according to video signals are formed into optical images varying in transmittance by the action of the incident side polarizing plates 18, 19 and 20, and emitting side polarizing plates 24, 25 and 26. Light rays emitted from the liquid crystal panels 21, 22 and 23 are combined into one light by a color combining optical system constituted by dichroic mirrors 27 and 28 and a flat mirror 29. The combined light is incident upon a projection lens 30, and the optical images on the three liquid crystal panels 21, 22 and 23 are projected onto a projection screen (not shown) through magnification by the lens 30.
FIG. 24 shows the construction of a conventional TFT (thin film transistor: referred to as TFT hereinafter) liquid crystal panel.
In FIG. 24, the TFT liquid crystal panel generally includes two sheets of glass substrates 41 and 42 disposed through a predetermined interval and sealed therearound by a sealing resin for defining a closed space therebetween, in which TN liquid crystal 43 is enclosed.
At the side of the liquid crystal layer 43 of the incident side glass substrate 42, a common electrode 44 of a transparent conductive film is provided, while at the side of the liquid crystal layer 43 of the emitting side glass substrate 42, pixel electrode 45 by a transparent conductive film are formed in a matrix pattern, with TFT 46 being formed in the vicinity of the respective pixel electrodes 45 as switching elements. On the common electrode 44 and the pixel electrode 45, alignment films for orientating the TN liquid crystal in a predetermined state are formed. At the incident side and the emitting side of the liquid crystal panel, the polarizing plates 47 and 48 are disposed, with absorbing axes thereof directed in a predetermined direction. In order to prevent erroneous functions of the TFT 46 by the intense light incident upon the liquid crystal panel, black matrixes 49 of metallic thin films are formed at the side of the liquid crystal layer 43 of the incident side glass substrate 41 for shielding the TFT 46 and wirings against light. Upon impression of signal voltages to the respective pixels through the TFT 46, the rotatory polarization of the liquid crystal layer for the respective pixels is varied, whereby the transmittance of each of the pixels can be controlled by the action of the two polarizing plates 47 and 48. Thus, the images as the variation of the transmittance are displayed on the liquid crystal panel.
Incidentally, light to be utilized by the TFT liquid crystal panel in the construction as shown in FIG. 24 is limited to the light transmitted through the black matrix 49, and brightness of the projected images is proportional to the aperture ratio (i.e., ratio of the area for all the openings 50 of the black matrix 49 to a total area of the display region) of the liquid crystal panel. If light incident upon the non-opening portions 51 of the black matrix 49 can also be utilized, the projected images may be made brighter, with an improved energy utilizing efficiency. Accordingly, there has also been conventionally disclosed, for example, in U.S. Pat. No. 5,052,783 to Hamada et al., a method for brightening projected images by disposing a lens array plate close to the incident side of the liquid crystal panel.
FIG. 25 shows an example of conventional light valve apparatuses in which a lens array plate is combined with the liquid crystal panel.
In FIG. 25, a lens array plate 61 has a plurality of microlens elements 64 formed in a matrix pattern on a surface of a transparent substrate 62 at the side of the liquid crystal panel 63. The lens array plate 61 is disposed close to the liquid crystal panel 63 so that the microlens elements 64 and the pixels 50 of the liquid crystal panel 63 correspond to each other. Light incident upon the lens array plate 61 is converted into a converged light by the microlens elements 64 and incident upon the pixel 50. Since light incident upon the non-opening portions 51 of the black matrix 49 is also incident upon the opening portions 50, the substantial aperture ratio of the liquid crystal panel 63 is improved, and the projected images are further brightened.
For obtaining very fine projected images by the construction as shown in FIG. 25, it may be so arranged to increase the number of pixels of the liquid crystal panel. If the dimensions of the display screens of the liquid crystal panels are the same, the pixel pitch is to be decreased, in which case, however, problems as described hereinbelow will take place.
In the case where a lens array is employed, contracted real images of the light source are formed on the pixels 50 of the liquid crystal panel 63. When the size of such real images is larger than that of the pixel 50, although the substantial aperture ratio may be improved upon incidence of parallel light rays, the projected images will not be brightened. For reducing the size of the real image of the light source, a focal length of the microlens element 64 for the lens array plate 61 must be reduced, and for this purpose, it is necessary to make the glass substrate 41 at the incident side thinner. However, if the incident side glass substrate 41 is made thin, it becomes difficult to make the thickness of the liquid crystal layer 43 uniform. In order to overcome such an inconvenience, there has also been conventionally proposed, for example, in Japanese Patent Laid-Open Publication Tokkaihei No. 2-302726 by Ito et al., a method for disposing lens elements within an incident side glass substrate 41. In connection with the above, however, in the case where a lens array of a refractive index distribution type is prepared by the ion exchange method, it is required to employ a glass material containing alkali ion as the glass substrate, and in this case, there is a problem that characteristics of the TFT are undesirably deteriorated by the elution of the alkali ion. Meanwhile, when the lens array is formed between two glass substrates, materials different in the refractive index must be combined, and in this case, due to a difference in the thermal expansion coefficient, it is also difficult to make the thickness of the liquid crystal layer uniform in a broad temperature range. Anyhow, by the practice to form the microlens elements within the incident side glass substrate, it is difficult to display images at high quality on the liquid crystal panel. Thus, there has been a problem that after all, fine and bright projection images at high quality can not be readily obtained. Subsequently, with respect to a video camera, it is necessary to make the entire unit compact in size and light in weight for improving portability thereof, and employment of a liquid crystal panel for the view-finder is considered to reduce the size of the video camera as a whole. For making the view-finder compact and light weight, and displaying an image at high image quality on the liquid crystal panel, size of the display screen for the liquid crystal panel must be reduced, with an increase in the number of pixels. In other words, the pitch of the pixels for the liquid crystal panel must be decreased. In that case, however, the aperture ratio of the liquid crystal panel is made small, and thus, the displayed image is dark. Although a brighter light source may be employed for making the displayed image brighter, power consumption by the light source becomes large thereby, and continuous using time by one charging of a battery in undesirably reduced.
2. Description of the Prior Art